The invention generally relates to a technique and apparatus to control the transient response of a fuel cell system.
A fuel cell is an electrochemical device that converts chemical energy produced by a reaction directly into electrical energy. For example, one type of fuel cell includes a polymer electrolyte membrane (PEM), often called a proton exchange membrane, that permits only protons to pass between an anode and a cathode of the fuel cell. At the anode, diatomic hydrogen (a fuel) is reacted to produce hydrogen protons that pass through the PEM. The electrons produced by this reaction travel through circuitry that is external to the fuel cell to form an electrical current. At the cathode, oxygen is reduced and reacts with the hydrogen protons to form water. The anodic and cathodic reactions are described by the following equations:
H2xe2x86x922H++2exe2x88x92 at the anode of the cell, and
O2+4H++4exe2x88x92xe2x86x922H2O at the cathode of the cell.
A typical fuel cell has a terminal voltage near one volt DC. For purposes of producing much larger voltages, several fuel cells may be assembled together to form an arrangement called a fuel cell stack, an arrangement in which the fuel cells are electrically coupled together in series to form a larger DC voltage (a voltage near 100 volts DC, for example) and to provide more power.
The fuel cell stack may include flow plates (graphite composite or metal plates, as examples) that are stacked one on top of the other, and each plate may be associated with more than one fuel cell of the stack. The plates may include various surface flow channels and orifices to, as examples, route the reactants and products through the fuel cell stack. Several PEMs (each one being associated with a particular fuel cell) may be dispersed throughout the stack between the anodes and cathodes of the different fuel cells. Electrically conductive gas diffusion layers (GDLs) may be located on each side of each PEM to form the anode and cathodes of each fuel cell. In this manner, reactant gases from each side of the PEM may leave the flow channels and diffuse through the GDLs to reach the PEM.
A fuel cell system may include a fuel processor that converts a hydrocarbon (natural gas or propane, as examples) into a fuel flow for the fuel cell stack. For a given output power of the fuel cell stack, the fuel flow to the stack must satisfy the appropriate stoichiometric ratios governed by the equations listed above. Thus, a controller of the fuel cell system may determine the appropriate power that the stack needs to supply, and based on this determination, the controller estimates the fuel flow to satisfy the appropriate stoichiometric ratios to produce this power. In this manner, the controller regulates the fuel processor to produce this flow, and in response to the controller determining that a change in the output power is needed, the controller estimates a new rate of fuel flow and controls the fuel processor accordingly.
The fuel cell system may provide power to a load, such as a load that is formed from residential appliances and electrical devices that may be selectively turned on and off to vary the power that is demanded by the load. Thus, the power that is consumed by the load may not be constant, but rather the power may vary over time and abruptly change in steps. For example, if the fuel cell system provides power to a house, different appliances/electrical devices of the house may be turned on and off at different times to cause the power that is consumed by the load to vary in a stepwise fashion over time.
It is possible that the fuel processor may not be able to adequately adjust its fuel flow output in a timely fashion to respond to a transient in the power that is consumed by the load. In this manner, the rate at which the power that is consumed by the load changes during a transient may be significantly faster than the rate at which the fuel processor can change its fuel output. For example, the time constant of the fuel processor may be in the order of minutes, and the time constant at which the power that is consumed by the load changes during a transient may be in the order of seconds. Due to this discrepancy, it is possible that the output of the fuel processor may significantly lag transients in the power that is consumed by the load, thereby resulting in inefficient operation of the fuel cell system.
For example, if the fuel cell system powers a house, one or more appliances may be briefly turned on to momentarily increase the power that is consumed by the appliance(s) to produce a transient. However, by the time the fuel processor responds to counteract this increase, the one or more appliances that were turned on may have been turned off. During the time during which the fuel processor falls behind, it is possible that power from a power grid may provide the power (to the load) that the fuel cell system fails to provide. However, this arrangement may contribute to increased costs associated with powering the load.
Thus, there is a continuing need for an arrangement and/or technique to address one or more of the problems that are stated above.
In an embodiment of the invention, a technique that is usable with a fuel cell stack includes coupling the fuel cell stack to a load, monitoring a power that is consumed by the load and determining if the power is increasing or decreasing. If the output power is increasing, a first control technique is used to control a fuel flow to the fuel cell stack, and if the output power is decreasing, a second control technique that is different from the first control technique is used to control the fuel flow to the stack.
Advantages and other features of the invention will become apparent from the following description, from the drawing and from the claims.